1. Field of the Invention
The present invention relates generally to improved apparatus and methods for dither signal generation in audio circuits to prevent idle tones, and includes particular applications of these circuits in digital modulators of a type useful in high fidelity audio processing.
2. Related Art
Dither signals are commonly generated in audio circuits to overcome the tendency of high-gain feedback amplification circuits to generate audible output tones (referred to as idle tones) during periods of low or zero input amplitude when the output should be low or zero. Dither signals in the form of white noise are typically introduced into the feedback circuit during periods of low input amplitude. However, this dithering function introduces a small but measurable amount of noise into the circuit and therefore reduces signal-to-noise ratios.
Digital-to-analog converters (DACs) are used to process digital audio signals. Typically digital data signals are received from a digital replay device or over a network, such as a cable television network. The signals are then processed by a DAC in an audio amplifier, cable receiver, or other audio device to produce an analog output within a frequency range that, when connected to a transducer such as a speaker, generates human audible sounds.
DACs used in high-fidelity audio processing typical include digital modulators that convert highly over-sampled digital values from high precision (16-20 bits) to low precision (1-3 bits), with the objective of substantially eliminating noise from the human audible band.
To prepare these low precision signals for conversion to analog form, they are mapped into digital sequences to prevent parasitic elements from degrading the signal. An analog signal is then generated from the mapped digital signal and transmitted to audio reproduction equipment.
Known DACs are susceptible to various types of signal distortion, harmonics, dependency on past output, and generation of unwanted output at low signal input levels. These circuits achieve high fidelity output through high-precision digital signal processing, and in this context degradation due to the introduction of dithering signals is a component of overall signal distortion. Therefore, improvement in dither signal generation to effect reductions of in-band noise component is desirable.
An improved dither generation circuit and method for digital audio circuits generates pseudo-random numbers that are then interpreted as 2""s complement numbers representing data points of a bipolar signal. The random number signal is high-pass filtered to reduce the energy contained in the audio band. The resulting dither signal is applied to the circuit in its main feedback loop and is effective to reduce or eliminate idle tones. Because of its spectrally shaped characteristic this dither signal introduces less noise into the audio band of interest and thereby improves the overall signal-to-noise ratio of the audio circuit.
These improved dithering methods and circuits are particularly useful in the context of high-fidelity digital modulator circuits, and in combination with other novel features developed by the same inventor for such circuits. As disclosed herein, the improved dithering methods are optionally and advantageously combined with particular improvements in mapping the output of a quantizer receiving the dither signals, and by including the mapping function in the circuit""s main feedback loop.